Some disk drive devices for computer systems have a recordable disk driven by a spindle motor. To conduct read and write operations, the recordable disk is rotated at a predetermined rotational speed, and a reading or writing element reads or writes information to or from the recordable disk. To determine, monitor, and adjust the speed of the recordable disk, these disk drives may utilize an encoder wheel configured to rotate at the same speed as the recordable disk.
The encoder wheel may be a disk-shaped device that includes a series of reflective and non-reflective regions alternatively disposed circumferentially around the encoder wheel. A directed light source shines light on the encoder wheel as it rotates with the recordable disk. When the light shines on the reflective regions, a relatively great amount of the light is reflected back to a photodetector. Conversely, when the light shines on the non-reflective regions, a relatively low amount of light (if any) is reflected back to the photodetector. By using the photodetector to determine when the light is reflecting off of a reflective region or is reflecting off of a non-reflective region, the actual rotational speed of the encoder wheel can be determined by measuring the elapsed time of reflective and non-reflective regions of the encoder wheel, respectively, passing by the photodetector. Because the encoder wheel is configured to rotate at the same speed as the recordable disk, it is assumed that the determined rotational speed of the decoder is the same as the actual rotational speed of the recordable disk. The determined rotational speed of the decoder can be compared to a predetermined desired rotational speed of the recordable disk, and, if they differ, the spindle motor can be controlled to adjust the actual rotational speed of the recordable disk upward or downward, as appropriate, to match the predetermined desired speed of the recordable disk.
While this type of feedback system provides a suitable means of ensuring that the recordable disk rotates at the predetermined rotational speed, errors in determining the actual rotational speed of the recordable disk may result if the center of the disk-shaped encoder wheel is not properly aligned with the center of the recordable disk. Specifically, if the center of the disk-shaped encoder wheel is misaligned with the center of the recordable disk, then the speed at which reflective/non-reflective regions of the encoder wheel pass over the photodetector will vary sinusoidally, even though the rotational speed of the recordable disk is constant. This creates a perceived sinusoidal error in the control signal that, if used as a feedback signal in the system, would cause an unintended sinusoidal speed variation of the recordable disk. The present invention was developed in light of these and other drawbacks.